Iron is essential nutrient for the growth of the most pathogenic microorganisms. However in vivo iron is complexed with host proteins such as transferrin in the blood and lactoferrin in secretions so that it is not available as a free ionic iron. Restriction in the availability of free iron in the host, the so-called nutritional immunity plays a key role in nonspecific defence strategy against potential pathogens. To overcome the lack of free iron, microorganisms produce substances that chelate iron and they are called siderophores. The outcome of every infection is therefore dependent on both the level of free iron present in the host and the efficiency of siderophore-mediated iron uptake system of the pathogen.